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usually hang up (crash) if there is no flow leaving a pond because (we believe) it <br />cannot iterate for normal flow depth in the downstream channel, forcing the user to <br />abandon the file and start over. This may be a glitch in the program and it may be <br />fixed in future releases, but it is significant operational problem in complex watersheds <br />such as these. Fortunately the problem can be managed in the current release of <br />SEDCAD by use of the generic "relative elevation" stock tanks. <br />The typical generic future stock tank is nominally 10 feet deep and assumed to be <br />prismatic at the time the modeling is performed. We use relative elevations and set <br />the pond bottom at El 90 and the overflow spillway at EI 100, and then input the area <br />values at 2 foot intervals to allow SEDCAD to produce a stage -storage curve. The <br />technique of using relative elevations and simple pond geometry then allows us to <br />tweak the starting pool elevations while limiting the chances for a crash. As suggested <br />above, the program will crash if ever it has no flow out of any one of these ponds. <br />Since this technique has been used in numerous approved technical revisions, we <br />request that it continue to be allowed here. No changes have been made in response <br />to this comment. <br />S. In Exhibit 7, Item 20, Part E, the subwatersheds for the West Taylor West Tributary Ditch <br />should not straddle the ditch to make each one have a homogeneous curve number. Rather, a given <br />subwatershed should be on only one side of the ditch, and the curve number should be determined <br />from an area -weighted average. Please review the way the subwatersheds were drawn and remodel <br />the West Taylor West Tributary Ditch. <br />Response: Exhibit 7, Item 20, Part E does not provide typical figures showing modeling <br />parameters; instead it refers the reader to figures contained in Exhibit 7-20C. <br />One of the many benefits of SEDCAD is that it can easily handle a network of multiple <br />watersheds with subwatersheds within in them, each with individual hydrologic parameters <br />while still accurately calculating and combining the peak flows and total runoff volumes. This is <br />not the case with older hydrology programs that could only handle a single watershed and <br />required the use of area -weighted curve numbers. We do not believe that weight -averaging <br />curve numbers is appropriate. <br />The drainage boundaries used for the West Taylor West Tributary Ditch appears to be <br />reasonable. For example, about Station 40+00, there are two undisturbed areas (SW -8-1 and <br />SW -10-2), three watersheds of reclamation modeled at curve numbers of 62 and 74, and one <br />watershed of topsoiled and seeded land modeled with a curve number of 80 (SW -10-1). The <br />amount of runoff resulting from a given curve number is quite non-linear. We always apply <br />specific curve numbers to a given condition as outlined in the discussion provided in the <br />Introductory text of Volume 2D entitled Methodologies and Assumptions for Sediment Pond <br />Design Evaluations and have never averaged curve numbers in any of the SEDCAD models <br />we are familiar with. <br />Page 10 <br />